Chemically strengthened glass

ABSTRACT

The present invention relates to a chemically strengthened glass satisfying A1 [MPa] of 600 or more, A2 [MPa] of 50 or more, B1 [μm] of 6 or less, B2 [μm] of 10% or more of t [μm], C [MPa] of −30 or less, and A1/B1 [MPa/μm] of 100 or more when the chemically strengthened glass has a thickness t [μm] and a profile of a stress value [MPa] at a depth x [μm] from a glass surface is approximated by an error least-squares method in a region of 0&lt;x&lt;3t/8 using the following function while defining a compressive stress as positive and a tensile stress as negative: A1erfc(x/B1)+A2erfc(x/B2)+C, in which erfc is a complementary error function, and relations of A1&gt;A2 and B1&lt;B2 are satisfied.

TECHNICAL FIELD

The present invention relates to a chemically strengthened glass.

BACKGROUND ART

In recent years, a cover glass including a chemically strengthened glasshas been used for enhancing protection and beauty of a display device ofa mobile device such as a mobile phone, a smart phone, a personaldigital assistant (PDA), and a tablet terminal. The chemicallystrengthened glass is a glass in which a surface layer is formed on aglass surface by means of ion exchange for the purpose of increasing astrength of the glass. The surface layer includes at least a compressivestress layer which exists on the glass surface side and in which acompressive stress is generated owing to ion exchange, and a tensilestress layer which exists adjacent to the compressive stress layer onthe glass internal side and in which a tensile stress is generated. Thestrength of the chemically strengthened glass strongly depends on acompressive stress value (hereinafter referred to as “stress profile”)in which a depth from the glass surface is a variable.

The chemically strengthened glass for a cover glass is required suchthat it is not cracked when dropping on the plane of sand, asphalt, orthe like. When the chemically strengthened glass drops on the sand orasphalt and is cracked, protruding objects existing on the sand orasphalt stick into the glass, and a position deeper than the glasssurface becomes a starting point of cracking. Therefore, there is atendency that as a depth of the compressive stress layer (DOL) is large,the chemically strengthened glass is hardly cracked.

In addition, the chemically strengthened glass for a cover glass isrequired such that it is not cracked when deflected by an externalforce. When the chemically strengthened glass is cracked due todeflection, the starting point of cracking is the surface of the glass,and therefore, there is a tendency that as the compressive stress(value) (CS) on the glass surface is high, the chemically strengthenedglass is hardly cracked.

For example, PTL 1 describes a glass article utilizing a two-stagechemical strengthening treatment. PTL 1 proposes a method in which byusing a KNO₃/NaNO₃ mixed salt having a relatively low K concentrationfor the first-stage chemical strengthening, and by using a KNO₃/NaNO₃mixed salt having a relatively high K concentration for the second-stagechemical strengthening, both large DOL and high CS are made compatiblewith each other.

CITATION LIST Patent Literature

PTL 1: US 2015/0259244 A

SUMMARY OF INVENTION Technical Problem

However, in the glass article described in PTL 1, the strength of thechemically strengthened glass may be insufficient. It can be consideredthat this is caused due to insufficiency of the surface compressivestress and the depth of the compressive stress layer. On the other hand,to maintain a balance with the compressive stress on the glass surface,an internal tensile stress (CT) is generated inside the chemicallystrengthened glass, and as CS or DOL is large, CT becomes large. Whenthe chemically strengthened glass with large CT is cracked, the crackingmanner is vigorous with a large number of fragments, and as a result,the risk that the fragments scatter increases. Accordingly, it isdesired that the total amount of the compressive stresses is a certainvalue or less. In consequence, an object of the present invention is toprovide a chemically strengthened glass having a high surfacecompressive stress and a large depth of a compressive stress layer ascompared with those in the conventional art, while controlling the totalamount of the compressive stress values to a certain value or less.

Solution to Problem

The present inventors have made extensive and intensive investigations.As a result, it has been found that in a chemically strengthened glasshaving a two-stage stress distribution pattern including a stressdistribution pattern 1 (hereinafter also abbreviated as “pattern 1”)appearing on the glass surface side and a stress distribution pattern 2(hereinafter also abbreviated as “pattern 2”) appearing on the glassinternal side, by making a depth of a compressive stress layer of thepattern 1 small and making a compressive stress value of the pattern 2small, a high surface compressive stress and a large depth of thecompressive stress layer can be attained while controlling the totalamount of the compressive stress values to a certain value or less,thereby leading to accomplishment of the present invention.

Specifically, the present invention relates to the following <1> to <7>.

<1> A chemically strengthened glass satisfying A_(i) [MPa] of 600 ormore, A₂ [MPa] of 50 or more, B₁ [μm] of 6 or less, B₂ [μm] of 10% ormore of t [μm], C [MPa] of −30 or less, and A₁/B₁ [MPa/μm] of 100 ormore when the chemically strengthened glass has a thickness t [μm] and aprofile of a stress value [MPa] at a depth x [μm] from a glass surfaceis approximated by an error least-squares method in a region of 0<x<3t/8using the following function (I) while defining a compressive stress aspositive and a tensile stress as negative:

A₁erfc(x/B₁)+A₂erfc(x/B₂)+C  (I),

in which erfc is a complementary error function, and relations of A₁>A₂and B₁<B₂ are satisfied.

<2> The chemically strengthened glass as set forth in <1>, wherein B₂[μm] is 20% or more of t [μm].<3> The chemically strengthened glass as set forth in <1> or <2>,wherein A₂ [MPa] is 150 or more, and A₂/B₂ [MPa/μm] is 4 or less.<4> The chemically strengthened glass as set forth in any one of <1> to<3>, wherein C [MPa] is −70 or less.<5> The chemically strengthened glass as set forth in any one of <1> to<4>, wherein the thickness t is 0.3 mm or more and 2 mm or less.<6> The chemically strengthened glass as set forth in any one of <1> to<5>, having a base composition including 50 to 80% of SiO₂, 4 to 30% ofAl₂O₃, 0 to 15% of B₂O₃, 0 to 15% of P₂O₅, 0 to 20% of MgO, 0 to 20% ofCaO, 0 to 10% of SrO, 0 to 10% of BaO, 0 to 10% of ZnO, 0 to 10% ofTiO₂, 0 to 10% of ZrO₂, 3 to 20% of Li₂O, 0 to 20% of Na₂O, and 0 to 20%of K₂O in mole percentage on an oxide basis.<7> The chemically strengthened glass as set forth in any one of <1> to<5>, having a base composition including 50 to 80% of SiO₂, 4 to 30% ofAl₂O₃, 0 to 15% of B₂O₃, 0 to 15% of P₂O₅, 0 to 20% of MgO, 0 to 20% ofCaO, 0 to 10% of SrO, 0 to 10% of BaO, 0 to 10% of ZnO, 0 to 10% ofTiO₂, 0 to 10% of ZrO₂, 3 to 20% of Li₂O, 0 to 20% of Na₂O, 0 to 20% ofK₂O, and 0.1 to 5% of Y₂O₃ in mole percentage on an oxide basis.<8> The chemically strengthened glass as set forth in any one of <1> to<7>, which is a glass substrate for a cover glass.

Advantageous Effects of Invention

The chemically strengthened glass of the present invention has a highsurface compressive stress and a large depth of a compressive stresslayer as compared with those in the conventional art, and also has thetotal amount of the compressive stresses of a certain value or less anda high strength.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an example of a stress distribution pattern of achemically strengthened glass of each of a working example and acomparative example. A solid line of FIG. 1 is a stress profile of achemically strengthened glass described in Example 5 in Table 1, and adotted line is a stress profile of a chemically strengthened glassdescribed in Example 15 in Table 1.

FIG. 2 is a graph illustrating a correlation between a depth (B₁) of astress distribution pattern 1 on the glass surface side and acompressive stress (CS) of the outermost surface. B₁ and CS in FIG. 2are plots of values described in Examples 1 to 19 in Table 1, in whichplots of white triangles are concerned with comparative examples(Examples 13 to 19), and plots of black circles are concerned withworking examples (Examples 1 to 12).

FIG. 3 illustrates an example of a stress distribution pattern of achemically strengthened glass of a working example (Example 20).

FIG. 4 illustrates an example of a stress distribution pattern of achemically strengthened glass of a working example (Example 21).

FIG. 5 illustrates an example of a stress distribution pattern of achemically strengthened glass of a working example (Example 22).

FIG. 6 illustrates an example of a stress distribution pattern of achemically strengthened glass of a working example (Example 23).

FIG. 7 illustrates an example of a stress distribution pattern of achemically strengthened glass of a working example (Example 24).

DESCRIPTION OF EMBODIMENTS

The chemically strengthened glass of the present invention is hereunderdescribed in detail. In addition, in this description, the numericalrange expressed using “to” indicates a range including the numericalvalues before and after “to” as the minimum value and the maximum value,respectively. In this specification, the terms “to” are hereunder usedas the same meanings unless otherwise indicated.

The chemically strengthened glass of the present invention is achemically strengthened glass satisfying all of the items (1) to (6)shown below when the chemically strengthened glass has a thickness t[μm] and a profile of a stress value [MPa] at a depth x [μm] from aglass surface is approximated by an error least-squares methodespecially in a region of 0<x<3t/8 using the following function (I)while defining a compressive stress as positive and a tensile stress asnegative.

A₁erfc(x/B₁)+A₂erfc(x/B₂)+C  (I)

In the function (I), erfc is a complementary error function, andrelations of A₁>A₂, and B₁<B₂ are satisfied.

(1) A₁ [MPa] is 600 or more;

(2) A₂ [MPa] is 50 or more;

(3) B₁ [μm] is 6 or less;

(4) B₂ [μm] is 10% or more of t [μm];

(5) C [MPa] is −30 or less; and

(6) A₁/B₁ [MPa/μm] is 100 or more.

The chemically strengthened glass of the present invention includes acompressive stress layer formed by a chemical strengthening treatment onthe surface thereof. In the chemical strengthening treatment, thesurface of the glass is ion-exchanged to form a surface layer having acompressive stress remaining therein. Specifically, an alkali metal ion(typically, a Li ion or a Na ion) having a small ionic radius existingin the vicinity of the glass sheet surface is substituted with an alkaliion (typically, a Na ion or a K ion for the Li ion, and a K ion for theNa ion) having a larger ionic radius by ion exchange at a temperatureequal to or lower than a glass transition point. According to this, acompressive stress remains on the surface of the glass, and a strengthof the glass is improved.

The chemically strengthened glass obtained through a two-stage chemicalstrengthening treatment (hereinafter also referred to as “two-stagestrengthening”) in which after a first-stage chemical strengtheningtreatment, a second-stage chemical strengthening treatment is performedunder a condition different from a first-stage condition (e.g., the kindof a salt, time, etc.) has a two-stage stress distribution patternincluding a stress distribution pattern 1 on the glass surface side anda stress distribution pattern 2 on the glass internal side. An exampleof such a stress distribution pattern of the chemically strengthenedglass is illustrated in FIG. 1.

With respect to a profile of a stress value [MPa] at a depth x [μm] fromthe glass surface, a stress distribution pattern 1 on the glass surfaceside and a stress distribution pattern 2 on the glass internal side canbe approximated using the following function (a) and the followingfunction (b), respectively, while defining a compressive stress aspositive and a tensile stress as negative.

A₁erfc(x/B₁)+C₁  (a)

A₂erfc(x/B₂)+C₂  (b)

In the functions (a) and (b), erfc is a complementary error function,and relations of A₁>A₂, and B₁<B₂ are satisfied.

A₁ is a parameter expressing the degree of a compressive stress of thestress distribution pattern 1; B₁ is a parameter expressing the depth ofa compressive stress layer of the stress distribution pattern 1; A₂ is aparameter expressing the degree of a compressive stress of the stressdistribution pattern 2; and B₂ is a parameter expressing the depth of acompressive stress of the stress distribution pattern 2. The sum of C₁and C₂ is corresponding to the degree of an internal tensile stress, andthe sum of A₁, A₂, C₁, and C₂ is corresponding to a compressive stress(CS) of the outermost surface. The present inventors have advancedresearch regarding the chemically strengthened glass having such atwo-stage stress distribution pattern and have found that there is atendency that as the depth (B₁) of the compressive stress layer of thepattern 1 on the glass surface side is small, the compressive stress(CS) on the outermost surface becomes large, as illustrated in FIG. 2.

Furthermore, the present inventors have found that the chemicallystrengthened glass satisfying all of the conditions (1) to (6) shownbelow when the chemically strengthened glass has a thickness t [μm] anda profile of a stress value [MPa] at a depth x [μm] from the glasssurface is approximated by an error least-squares method in a region of0<x<3t/8 according to the foregoing function (I) using the foregoingfunctions (a) and (b) while defining a compressive stress as positiveand a tensile stress as negative, has both a high surface compressivestress and a large depth of a compressive stress layer as compared withthose in the conventional art, and makes it possible to control thetotal amount of the compressive stresses to a certain value or less andhas a high strength.

(1) A₁ [MPa] is 600 or more;

(2) A₂ [MPa] is 50 or more;

(3) B₁ [μm] is 6 or less;

(4) B₂ [μm] is 10% or more of t [μm];

(5) C [MPa] is −30 or less; and

(6) A₁/B₁ [MPa/μm] is 100 or more.

With respect to the foregoing (1), A₁ [MPa] is 600 or more, preferably700 or more, more preferably 750 or more, and still more preferably 800or more. In the case where A₁ [MPa] is 600 or more, a high strengthagainst deflection or bending is obtained. Although an upper limit of A₁[MPa] is not particularly limited, in the case where the compressivestress is large, the internal tensile stress to be generated accordingthereto tends to become large, and in the case where the internaltensile stress becomes large, when a flaw generated on the glass surfacereaches the inside, a risk for vigorous fracture becomes large. Then, inorder to enhance safety at the time of fracture, A₁ [MPa] is preferably1,000 or less, more preferably 950 or less, and still more preferably920 or less.

With respect to the foregoing (2), A₂ [MPa/μm] is 50 or more, morepreferably 100 or more, still more preferably 150 or more, andespecially preferably 200 or more. In the case where A₂ [MPa/μm] is 50or more, for example on the occasion of dropping on the sand, crackresistance with flawing due to an acute-angle object is improved.Although an upper limit of A₂ is not particularly limited, in order toenhance safety at the time of fracture, A₂ is preferably 600 or less,more preferably 500 or less, and still more preferably 400 or less.

With respect to the foregoing (3), B₁ [μm] is 6 or less, preferably 5.5or less, more preferably 5.0 or less, and still more preferably 4.5 orless. In the case where B₁ [μm] is 6 or less, it becomes possible toimprove CS while suppressing an integrated value of the compressivestress small. Although a lower limit of B₁ [μm] is not particularlylimited, since it is preferred that B₁ is thoroughly larger than thedepth of a flaw existing on the glass surface or the surface roughness,B₁ is preferably 0.5 or more, more preferably 0.75 or more, and stillmore preferably 1.0 or more.

With respect to the foregoing (4), B₂ [μm] is 10% or more, preferably20% or more, more preferably 21% or more, still more preferably 22% ormore, and especially preferably 23% or more of t [μm]. In the case whereB₂ [μm] is 10% or more of t [μm], for example on the occasion ofdropping on the sand, crack resistance with flawing due to anacute-angle object is improved. Although an upper limit of the ratio ofB₂ [μm] to t [μm] is not particularly limited, in view of the fact thatan integrated value of the compressive stress value must be controlledto a certain value or less, B₂ [μm] is preferably 40% or less, morepreferably 35% or less, and still more preferably 33% or less.

With respect to the foregoing (5), C is −30 or less. C is a constantrelative to the internal tensile stress of the glass, and in the casewhere C is larger than −30, it is difficult to make the compressivestress value of the glass surface layer sufficiently high. C ispreferably −50 or less, more preferably −70 or less, and still morepreferably −80 or less. In addition, C is preferably −150 or more, morepreferably −140 or more, and still more preferably −130 or more. In thecase where C is −150 or more, when the glass is fractured, the matterthat the cracking manner is vigorous with a large number of fragments isprevented from occurring, and as a result, the risk that the fragmentsscatter can be reduced.

With respect to the foregoing (6), A₁/B₁ [MPa/μm] is 100 or more,preferably 150 or more, more preferably 180 or more, and still morepreferably 220 or more. In the case where A₁/B₁ [MPa/μm] is 100 or more,CS can be increased while suppressing an integrated value of thecompressive stress value low. Although an upper limit of A₁/B₁ [MPa/μm]is not particularly limited, when the respective preferred ranges of A₁and B₁ are taken into consideration, A₁/B₁ [MPa/μm] is preferably 2,000or less, more preferably 1,500 or less, and still more preferably 900 orless.

In the chemically strengthened glass of the present invention, A₂[MPa/μm] is preferably 150 or more, more preferably 175 or more, stillmore preferably 200 or more, and especially preferably 250 or more, andA₂/B₂ [MPa/μm] is preferably 4 or less, more preferably 3.5 or less,still more preferably 3 or less, and especially preferably 2.5 or less.In the case where A₂ [MPa/μm] is 150 or more and A₂/B₂ [MPa/μm] is 4 orless, for example on the occasion of dropping on the sand, the crackresistance with flawing due to an acute-angle object is more improved.Although a lower limit of A₂/B₂ [MPa/μm] is not particularly limited,when the respective preferred ranges of A₂ and B₂ are taken intoconsideration, A₂/B₂ [MPa/μm] is preferably 0.9 or more, more preferably1.0 or more, and still more preferably 1.1 or more.

A₁, B₁, A₂, and B₂ can be adjusted by adjusting the condition of thechemical strengthening treatment, the composition of the glass, and thelike.

However, in the production of the chemically strengthened glass of thepresent invention, it is preferred to make the depth of a compressivestress layer for forming the stress distribution pattern 1 on the glasssurface side small and to make the depth of a compressive stress layerfor forming the stress distribution pattern 2 on the glass internal sidelarge. Therefore, it is preferred to make an ion exchange rate forforming the pattern 1 low and to make an ion exchange rate for formingthe pattern 2 high.

In the conventional chemically strengthened glasses, it is general thatboth of the pattern 1 and the pattern 2 are formed through ion exchangebetween a sodium ion in the glass and a potassium ion in a strengtheningsalt, and the ion exchange rate has been principally controlled by thechemical strengthening temperature.

But, according to a method of controlling the ion exchange rate bycontrolling the chemical strengthening temperature, there is a casewhere the chemical strengthening temperature can be controlled only in acertain temperature range due to restrictions in the melting point andthe decomposition temperature of the strengthening salt. That is, whenthe chemical strengthening temperature is excessively low, thestrengthening salt is hardly melted, so that the chemical strengtheningbecomes difficult, whereas when the chemical strengthening temperatureis excessively high, the strengthening salt is thermally decomposedstep-by-step, so that the production of a stable chemically strengthenedglass may become difficult.

Then, the present inventors have found that in addition to control ofthe chemical strengthening temperature, by using a glass having apreferred specified glass composition, the preferred stress distributionpattern is easily obtained, and a high surface compressive stress and alarge depth of a compressive stress layer can be attained whilecontrolling the total amount of the compressive stress values to acertain value or less, an aspect of which is the problem of the presentinvention. A preferred glass composition is described later.

The surface compressive stress (value) (CS) [MPa] is a value measuredwith a surface stress meter (for example, a surface stress meterFSM-6000, manufactured by Orihara Industrial Co., Ltd.).

Preferably, the chemically strengthened glass of the present inventionhas a surface compressive stress value (CS) of 600 MPa or more. CS ofthe chemically strengthened glass of 600 MPa or more is preferredbecause the chemically strengthened glass has a favorable strength as acover glass of a smart phone or a tablet PC. CS of the chemicallystrengthened glass is more preferably 700 MPa or more, still morepreferably 800 MPa or more, especially preferably 850 MPa or more, andmost preferably 900 MPa or more.

On the other hand, although an upper limit of CS of the chemicallystrengthened glass is not particularly limited, there is a tendency thatwhen CS becomes large, the internal tensile stress becomes large, sothat there is a concern that the glass is vigorously fractured.Therefore, from the viewpoint of safety at the time of fracture, CS is,for example, 2,000 MPa or less, preferably 1,700 MPa or less, morepreferably 1,500 MPa or less, and still more preferably 1,300 MPa orless.

CS of the chemically strengthened glass can be appropriately adjusted byadjusting the condition of the chemical strengthening treatment, thecomposition of the glass, and the like.

In the chemically strengthened glass of the present invention, the depthof a compressive stress (DOL) is preferably 70 μm or more. In the casewhere DOL is 70 μm or more, for example on the occasion of dropping onthe sand, crack resistance with flawing due to an acute-angle object isimproved. In order to make the strength of the chemically strengthenedglass high, DOL is preferably 60 μm or more, and more preferablystepwise, 90 or more, 100 μM or more, 110 μm or more, 120 μm or more,130 μm or more, or 140 or more.

On the other hand, although an upper limit of DOL is not particularlylimited, in order to suppress an integrated value of the compressivestress value small, DOL is, for example, 200 μm or less, preferably 180μm or less, more preferably 170 μm or less, and especially preferably160 μm or less.

DOL can be appropriately adjusted by adjusting the condition of thechemical strengthening treatment, the composition of the glass, and thelike.

In this specification, DOL is a depth from the glass surface of aportion where the stress in the stress profile is zero. DOL can beestimated by thinning a cross section of the glass and analyzing thethinned sample using a birefringence imaging system. Examples of thebirefringence imaging system include a birefringence imaging systemAbrio-IM, manufactured by Tokyo Instruments, Inc. In addition, DOL canalso be estimated by utilizing scattered-light photoelasticity.According to this method, DOL can be estimated by making a lightincident from the surface of the glass and analyzing a polarized lightof the scattered light.

As for the chemically strengthened glass of the present invention, in afracture test according to an indenter indentation test under acondition of holding a load ranging from 5 kgf to 10 kgf for 15 secondswith a pyramidal diamond indenter having an indenter angle of the facingangle of 90°, the number of fragments generated within a size of 25mm×25 mm is preferably 100 or less. In the case where the number offragments (fragmentation number) in the fracture test by the indenterindentation test is 100 or less, even when the glass is fractured, highsafety can be ensured. The fragmentation number is more preferably 50 orless, still more preferably 20 or less, and especially preferably 10 orless.

As for the chemically strengthened glass of the present invention, in atest of colliding a diamond indenter (indenter angle of the facingangle: 160°) on the glass surface by using a pendulum impact tester, akinetic energy of the diamond indenter when a fragmentation probabilityof the glass is 50% (hereinafter referred to as “fracture energy”) ispreferably 80 mJ or more. In the pendulum impact test using a diamondindenter, in the case where the fracture energy is 80 mJ or more, whendropping on the plane having protruding objects, such as a sand or anasphalt, the glass is hardly cracked. The fracture energy is morepreferably 100 mJ or more, and still more preferably 120 mJ or more.

As for the chemically strengthened glass of the present invention, abending strength is preferably 500 MPa or more. In the case where thebending strength is 500 MPa or more, a sufficient strength againstdeflection that is assumed in daily life can be obtained. For example,it is known that when dropping on the plane having no protruding object,such as marble, the glass is cracked due to deflection. However, a glasshaving a bending strength of 500 MPa or more has a practicallysufficient strength when dropping on the plane having no protrudingobject.

As for the chemically strengthened glass of the present invention, abending strength after flawing as determined by the following method ispreferably 150 MPa or more. The degree of the tensile stress generatedon the cover glass surface at the time of dropping of a smart phone isabout 150 MPa, and in the case where the above-described bendingstrength is 150 MPa or more, fracture due to the stress generated bydropping can be prevented from occurring even after a flaw is generatedby an acute-angle object. The bending strength after flawing ispreferably 200 MPa or more, more preferably 250 MPa or more, and stillmore preferably 300 MPa or more.

The bending strength after flawing means a fracture stress value σa(bending strength, unit: MPa) obtained by performing a four-pointbending test under a condition of a lower spun of 30 mm, an upper spunof 10 mm, and a crosshead speed of 0.5 mm/min after the glass surface isflawed by pressing a diamond indenter (indenter angle of the facingangle: 110°) thereonto for 15 seconds at a load of 0.5 kgf.

Subsequently, the base composition of the chemically strengthened glassin the present invention is described. In this specification, the basecomposition of the chemically strengthened glass means the compositionof the glass before chemical strengthening (hereinafter sometimesreferred to as “base glass” or “glass for chemical strengthening”).Here, it can be considered that a portion having a tensile stress of thechemically strengthened glass (hereinafter also referred to as “tensilestress portion”) is a portion which is not ion-exchanged. Then, thetensile stress portion of the chemically strengthened glass has the samecomposition as the base glass, and the composition of the tensile stressportion can be regarded as the base composition.

The base composition of the chemically strengthened glass is describedbelow. The content of each component is expressed in mole percentage onan oxide basis unless otherwise stated.

The composition of the glass can be measured by a wet analysis methodsuch as an ICP emission spectroscopy. In the case where the glass doesnot contain a lot of components which are readily volatile especiallyduring the melting process of the glass, it is also possible todetermine the composition of the glass by a calculation from theblending ratio of the glass raw materials.

As the composition for the glass for chemical strengthening of thepresent invention (base composition of the chemically strengthened glassof the present invention), for example, one including 50 to 80% of SiO₂,4 to 30% of Al₂O₃, 0 to 15% of B₂O₃, 0 to 15% of P₂O₅, 0 to 20% of MgO,0 to 20% of CaO, 0 to 10% of SrO, 0 to 10% of BaO, 0 to 10% of ZnO, 0 to10% of TiO₂, 0 to 10% of ZrO₂, 3 to 20% of Li₂O, 0 to 20% of Na₂O, and 0to 20% of K₂O is preferred.

Furthermore, for example, one including 50 to 80% of SiO₂, 4 to 30% ofAl₂O₃, 0 to 15% of B₂O₃, 0 to 15% of P₂O₅, 0 to 20% of MgO, 0 to 20% ofCaO, 0 to 10% of SrO, 0 to 10% of BaO, 0 to 10% of ZnO, 0 to 10% ofTiO₂, 0 to 10% of ZrO₂, 3 to 20% of Li₂O, 0 to 20% of Na₂O, 0 to 20% ofK₂O, and 0.1 to 5% of Y₂O₃ in mole percentage on an oxide basis ispreferred.

According to the glass having the aforementioned composition, byadopting ion exchange between Na and K which has a low ion exchange ratefor forming the stress distribution pattern 1 on the glass surface side,and by adopting ion exchange between Na and Li which has a high ionexchange rate for forming the stress distribution pattern 2 on the glassinternal side, it becomes easy to make the depth of the compressivestress layer of the pattern 1 small and to make the depth of thecompressive stress layer of the pattern 2 large. The chemicalstrengthening treatment is mentioned later.

Specifically, for example, the following glasses are exemplified.

(a) A glass including 69 to 71% of SiO₂, 7 to 9% of Al₂O₃, 0 to 1% ofB₂O₃, 0 to 1% of P₂O₅, 7.5 to 9% of Li₂O, 4 to 6% of Na₂O, 0 to 2% ofK₂O, 6 to 8% of MgO, 0 to 1% of CaO, 0 to 1% of SrO, 0 to 1% of BaO, 0to 1% of ZnO, 0 to 1% of TiO₂, and 0 to 2% of ZrO₂.

(b) A glass including 62 to 69% of SiO₂, 8 to 12% of Al₂O₃, 0 to 1% ofB₂O₃, 0 to 1% of P₂O₅, 8 to 12% of Li₂O, 4 to 6% of Na₂O, 0 to 2% ofK₂O, 3 to 8% of MgO, 0 to 1% of CaO, 0 to 1% of SrO, 0 to 1% of Ba₀, 0to 1% of ZnO, 0 to 1% of TiO₂, 0 to 2% of ZrO₂, and 0.1 to 5% of Y₂O₃.

SiO₂ is a component that forms the network of the glass. In addition,SiO₂ is a component that enhances chemical durability and is a componentthat reduces the generation of a crack when the glass surface is flawed(indented). The content of SiO₂ is preferably 50% or more. The contentof SiO₂ is more preferably stepwise, 56% or more, 62% or more, 65% ormore, 67% or more, 68% or more, or 69% or more.

In order to prevent a decrease of meltability from occurring, thecontent of SiO₂ is preferably 80% or less, more preferably 75% or less,still more preferably 73% or less, especially preferably 72% or less,and most preferably 71% or less. However, in the case of containing 0.1%or more of Y₂O₃, the content of SiO₂ is preferably 74% or less, morepreferably 69% or less, and still more preferably 67% or less.

Al₂O₃ is a component that reduces the number of fragments when thechemically strengthened glass is cracked. In the case where the numberof fragments is small, the fragments are hardly scattered at the time offracture. In addition, Al₂O₃ is a component that is effective forimproving ion exchange performance at the time of chemical strengtheningand increasing the surface compressive stress after strengthening, andis also a component that increases Tg of the glass and increases theYoung's modulus.

The content of Al₂O₃ is preferably 4% or more, and more preferablystepwise, 4.5% or more, 5.0% or more, 5.5% or more, 6.0% or more, 6.5%or more, or 7.0% or more. However, in the case of containing 0.1% ormore of Y₂O₃, the content of Al₂O₃ is preferably 8% or more, morepreferably 8.5% or more, still more preferably 9.5% or more, andespecially preferably 10% or more.

The content of Al₂O₃ is preferably 30% or less, more preferably 15% orless, still more preferably 12% or less, especially preferably 10% orless, and most preferably 9% or less. In the case of containing 0.1% ormore of Y₂O₃, the content of Al₂O₃ is preferably 20% or less, morepreferably 18% or less, still more preferably 16% or less, especiallypreferably 14% or less, and yet still more preferably 12% or less. Inthe case where the content of Al₂O₃ is 30% or less, a decrease of acidresistance of the glass is suppressed, an increase of devitrificationtemperature is prevented from occurring, and an increase of viscosity ofthe glass is suppressed, thereby enabling the meltability to beimproved.

B₂O₃ is a component that improves chipping resistance of the glass forchemical strengthening or chemically strengthened glass and improves themeltability. Although B₂O₃ is not essential, in order to improve themeltability, the content in the case of containing B₂O₃ is preferably0.5% or more, more preferably 1% or more, and still more preferably 2%or more.

The content of B₂O₃ is preferably 15% or less, more preferably 10% orless, still more preferably 5% or less, especially preferably 3% orless, and most preferably 1% or less. In the case where the content ofB₂O₃ is 15% or less, the matter that striae is generated at the time ofmelting to decrease the quality of the glass for chemical strengtheningcan be prevented from occurring. In addition, in order to enhance theacid resistance, B₂O₃ is preferably not contained.

P₂O₅ is a component that improves ion exchange performance and chippingresistance. Although P₂O₅ may not be contained, the content in the caseof containing P₂O₅ is preferably 0.5% or more, more preferably 1% ormore, and still more preferably 2% or more.

The content of P₂O₅ is preferably 15% or less, more preferably 10% orless, still more preferably 5% or less, especially preferably 3% orless, and most preferably 1% or less. In the case where the content ofP₂O₅ is 15% or less, the number of fragments when the chemicallystrengthened glass is fractured can be reduced, and a decrease of acidresistance can be suppressed. In order to enhance the acid resistance,P₂O₅ is preferably not contained.

MgO is a component that increases the surface compressive stress of thechemically strengthened glass and is a component that reduces the numberof fragments when the chemically strengthened glass is fractured, andMgO may be contained. The content in the case of containing MgO ispreferably 2% or more, and more preferably stepwise, 3% or more, 4% ormore, 5% or more, or 6% or more.

The content of MgO is preferably 20% or less, and more preferablystepwise, 18% or less, 15% or less, 13% or less, 12% or less, 10% orless, 11% or less, or 8% or less. In the case where the content of MgOis 20% or less, devitrification when the glass for chemicalstrengthening is melted can be prevented from occurring.

CaO is a component that improves meltability of the glass for chemicalstrengthening and is a component that reduces the number of fragmentswhen the chemically strengthened glass is fractured, and CaO may becontained. The content in the case of containing CaO is preferably 0.5%or more, more preferably 1% or more, still more preferably 2% or moreand especially preferably 3% or more.

The content of CaO is preferably 20% or less, more preferably 14% orless, and still more preferably stepwise, 10% or less, 8% or less, 3% orless, or 1% or less. In the case where the content of CaO is 20% orless, a remarkable decrease of ion exchange performance can be preventedfrom occurring.

SrO is a component that improves meltability of the glass for chemicalstrengthening and is a component that reduces the number of fragmentswhen the chemically strengthened glass is fractured, and SrO may becontained. The content in the case of containing SrO is preferably 0.1%or more, more preferably 0.5% or more, and still more preferably 1.0% ormore.

The content of SrO is preferably 10% or less, more preferably 5% orless, still more preferably 3% or less, especially preferably 2% orless, and most preferably 1% or less. In the case where the content ofSrO is 10% or less, a remarkable decrease of ion exchange performancecan be prevented from occurring.

BaO is a component that improves meltability of the glass for chemicalstrengthening and is a component that reduces the number of fragmentswhen the chemically strengthened glass is fractured, and BaO may becontained. The content in the case of containing BaO is preferably 0.5%or more, more preferably 1% or more, and still more preferably 1.5% ormore.

The content of BaO is preferably 10% or less, more preferably 5% orless, still more preferably 3% or less, especially preferably 2% orless, and most preferably 1% or less. In the case where the content ofBaO is 10% or less, a remarkable decrease of ion exchange performancecan be prevented from occurring.

ZnO is a component that improves meltability of the glass, and ZnO maybe contained. The content in the case of containing ZnO is preferably0.25% or more, and more preferably 0.5% or more.

The content of ZnO is preferably 10% or less, more preferably 5% orless, still more preferably 3% or less, especially preferably 2% orless, and most preferably 1% or less. In the case where the content ofZnO is 10% or less, a remarkable decrease of weathering resistance ofthe glass can be prevented from occurring.

TiO₂ is a component that reduces the number of fragments when thechemically strengthened glass is fractured, and TiO₂ may be contained.The content in the case of containing TiO₂ is preferably 0.1% or more,and more preferably 0.15% or more.

The content of TiO₂ is preferably 10% or less, more preferably 5% orless, still more preferably 4% or less, yet still more preferably 3% orless, especially preferably 2% or less, and most preferably 0.25% orless. In the case where the content of TiO₂ is 10% or less,devitrification at the time of melting is hardly generated, and adecrease of the quality of the chemically strengthened glass can beprevented from occurring.

ZrO₂ is a component that increases the surface compressive stress whichis generated owing to ion exchange and has an effect for reducing thenumber of fragments when the glass for chemical strengthening isfractured, and ZrO₂ may be contained. The content in the case ofcontaining ZrO₂ is preferably 0.5% or more, and more preferably 1% ormore.

The content of ZrO₂ is preferably 10% or less, more preferably 8% orless, still more preferably 5% or less, especially preferably 3% orless, and most preferably 2% or less. In the case where the content ofZrO₂ is 10% or less, devitrification at the time of melting is hardlygenerated, and a decrease of the quality of the chemically strengthenedglass can be prevented from occurring.

Li₂O is a component that forms a surface compressive stress owing to ionexchange and is a component that reduces the number of fragments whenthe chemically strengthened glass is fractured. The content of Li₂O ispreferably 3% or more, more preferably 5% or more, still more preferably6% or more, especially preferably 7% or more, and typically 7.5% ormore. In the case of containing 0.1% or more of Y₂O₃, the content ofLi₂O is 1% or more, preferably 3% or more, more preferably 5% or more,still more preferably 7% or more, especially preferably 9% or more, andmost preferably 9.5% or more. In the case where the content of Li₂O is3% or more, it becomes possible to attain large DOL.

In order to suppress a devitrification growth rate at the time ofmelting of the glass, the content of Li₂O is preferably 20% or less,more preferably 15% or less, still more preferably 12% or less,especially preferably 11% or less, and most preferably 9% or less. Inthe case of containing 0.1% or more of Y₂O₃, the content of Li₂O is morepreferably 17% or less, still more preferably 15% or less, especiallypreferably 12% or less, and most preferably 11% or less. In the casewhere the content of Li₂O is 20% or less, a remarkable decrease of acidresistance of the glass can be prevented from occurring.

Na₂O is a component that forms a surface compressive stress layer owingto ion exchange and improves meltability of the glass. The content inthe case of containing Na₂O is preferably 1% or more. The content ofNa₂O is more preferably 2% or more, still more preferably 3% or more,and especially preferably 4% or more.

The content of Na₂O is preferably 20% or less, more preferably 9% orless, still more preferably 8% or less, especially preferably 7% orless, and most preferably 6% or less. In the case where the content ofNa₂O is 20% or less, a remarkable decrease of acid resistance of theglass can be prevented from occurring.

K₂O may be contained for the purpose of improving ion exchangeperformance, or the like. The content in the case of containing K₂O ispreferably 0.5% or more, more preferably 1% or more, still morepreferably 2% or more, and especially preferably 2.5% or more.

The content of K₂O is preferably 20% or less, more preferably 10% orless, still more preferably 5% or less, especially preferably 3% orless, and most preferably 2% or less. In the case where the content ofK₂O is 20% or less, an increase of the number of fragments when thechemically strengthened glass is fractured can be prevented fromoccurring.

Y₂O₃, La₂O₃, and Nb₂O₅ are each a component that reduces the number offragments when the chemically strengthened glass is fractured and may becontained. The each content in the case of containing each of thesecomponents is preferably 0.5% or more, more preferably 1% or more, stillmore preferably 1.5% or more, especially preferably 2% or more, and mostpreferably 2.5% or more.

In order to suppress devitrification on the occasion of forming theglass into a desired shape, 0.1% or more of Y₂O₃ is preferablycontained. The content of Y₂O₃ is more preferably 0.2% or more, stillmore preferably 0.5% or more, especially preferably 0.8% or more, andtypically 1% or more.

The content of each of Y₂O₃, La₂O₃, and Nb₂O₅ is preferably 6% or lessbecause the glass is hardly devitrified at the time of melting. Thecontent of each of Y₂O₃, La₂O₃, and Nb₂O₅ is more preferably 5% or less,still more preferably 4% or less, especially preferably 3% or less, andmost preferably 2% or less.

Ta₂O₅ and Gd₂O₃ may be each contained in a small amount in order toreduce the number of fragments when the chemically strengthened glass isfractured. However, since the refractive index or the reactanceincreases, the content thereof is preferably 1% or less, and morepreferably 0.5% or less, and still more preferably, they are notcontained.

Furthermore, for the use of the colored glass, a coloring component maybe added within a range where the achievement of desired chemicalstrengthening properties is not inhibited. Suitable examples of thecoloring component include Co₃O₄, MnO₂, Fe₂O₃, NiO, CuO, Cr₂O₃, V₂O₅,Bi₂O₃, SeO₂, TiO₂, CeO₂, Er₂O₃, and Nd₂O₃.

The content of the coloring component is preferably in a range of 7% orless in total in mole percentage on an oxide basis. In the case wherethe content of the coloring component is 7% or less, the matter that theglass is readily devitrified can be prevented from occurring. Thecontent of the coloring component is more preferably 5% or less, stillmore preferably 3% or less, and especially preferably 1% or less. In thecase where the visible ray transmittance of the glass takes preference,it is preferred that these components are substantially not contained.

In this specification, the wording “substantially not contained” meansthat such components are not contained except unavoidable impuritiescontained in raw materials and the like, i.e., they are notintentionally contained. Specifically, it is indicated that the contentin the glass composition is less than 0.1% by mol.

As a refining agent on the occasion of melting of the glass, SO₃, achloride, a fluoride, or the like may be appropriately contained.Preferably, As₂O₃ is not contained. In the case of containing Sb₂O₃, thecontent thereof is preferably 0.3% or less, and more preferably 0.1% orless, and most preferably, it is not contained.

The chemically strengthened glass of the present invention preferablyhas at least one selected from the group consisting of a sodium ion, asilver ion, a potassium ion, a cesium ion, and a rubidium ion on thesurface thereof. According to this, the compressive stress is inducedinto the surface, whereby the glass is highly strengthened. In addition,in the case where the silver ion is present on the surface,antibacterial properties can be imparted.

In the present invention, it is preferred to select a base compositionof the chemically strengthened glass such that DOL is 50 μm or more inthe case where an ion exchange treatment is performed for 1 hour with amolten salt at 400° C. including KNO₃, NaNO₃, or a mixed salt of KNO₃and NaNO₃ on a glass sheet having a thickness of 1 mm which has the basecomposition of the chemically strengthened glass and has been annealedunder the following condition. Here, the annealing is performed from atemperature T° C., which is 30° C. to 50° C. higher than the glasstransition temperature, to (T−300)° C. at a cooling rate of 0.5° C./min.

Furthermore, in the present invention, it is preferred to select a basecomposition of the chemically strengthened glass such that DOL is 70 μmor more in the case where an ion exchange treatment is performed for 1hour with a molten salt at 425° C. including KNO₃, NaNO₃, or a mixedsalt of KNO₃ and NaNO₃ on a glass sheet having a thickness of 1 mm whichhas the base composition of the chemically strengthened glass and hasbeen annealed under the following condition. Here, the annealing isperformed from a temperature T° C., which is 30° C. to 50° C. higherthan the glass transition temperature, to (T−300)° C. at a cooling rateof 0.5° C./min.

In the case of such a base composition, the ion exchange rate is high,and chemical strengthening can be performed for a short period of time.

From the viewpoint of making it possible to remarkably improve thestrength by chemical strengthening, the thickness (t) of the chemicallystrengthened glass of the present invention is preferably 2 mm or less,more preferably stepwise, 1.5 mm or less, 1 mm or less, 0.9 mm or less,0.8 mm or less, or 0.7 mm or less. In addition, from the viewpoint ofobtaining an effect of sufficient improvement of the strength by thechemical strengthening treatment, the foregoing thickness (t) ispreferably 0.3 mm or more, more preferably 0.5 mm or more, and stillmore preferably 0.6 mm or more.

The chemically strengthened glass of the present invention may have ashape other than the sheet form, for example, a fringed shape having adifferent thickness at outer periphery, depending upon the products,uses, and the like to which the glass is applied. In addition, theaforementioned glass sheet has two main surfaces and end surfaces thatneighbors them to form the sheet thickness, and the two main surfacesmay form flat faces that are parallel to each other. However, theconfiguration of the glass sheet is not limited thereto, and forexample, the two main surfaces may not be parallel to each other, or allor a part of one or both of the two main surfaces may be curved. Morespecifically, the glass sheet may be, for example, a glass sheet havinga warpage-free flat shape or may be a curved glass sheet having a curvedsurface.

The chemically strengthened glass of the present invention can be, for,example, produced in the following manner. The following productionmethod is an example in the case of producing a sheet-form chemicallystrengthened glass.

First of all, a glass to be subjected to a chemical strengtheningtreatment as mentioned later (glass for chemical strengthening) isprepared. For example, raw materials of respective components of theglass are blended and heated to melt in a glass melting furnace.Thereafter, the glass is homogenized by bubbling, stirring, addition ofa refining agent, and the like, and then formed into a glass sheet witha predetermined thickness according to a conventionally known method,followed by annealing.

Examples of the forming method of a glass includes a float method, apress method, a fusion method, and a down-draw method. In particular, afloat method suitable for mass production is preferred. In addition,other continuous forming methods than the float method, namely, a fusionmethod and a down-draw method are also preferred.

Thereafter, the formed glass is subjected to grinding and polishingtreatments, as the need arises, thereby forming a glass substrate. Inthe case where the glass substrate is cut into a predetermined shape andsize or chamfering of the glass substrate is performed, it is preferableto perform the cutting or chamfering of the glass substrate before thechemical strengthening treatment mentioned below, since a compressivestress layer is also formed on the end surface by the subsequentchemical strengthening treatment.

Then, after the formed glass substrate is subjected to the chemicalstrengthening treatment, the chemically strengthened glass of thepresent invention can be produced by washing and drying.

In the chemical strengthening treatment, by bringing the glass intocontact with a molten liquid of a metal salt (for example, potassiumnitrate) containing a metal ion having a large ionic radius (typically,a Na ion or a K ion) through immersion or the like, a metal ion having asmall ionic radius (typically, a Na ion or a Li ion) in the glass issubstituted with the metal ion having a large ionic radius.

In particular, in order to perform the chemical strengthening treatmentat a high ion exchange rate, it is preferred to substitute the Li ion inthe glass with the Na ion (Li—Na substitution).

Examples of the molten salt for performing the chemical strengtheningtreatment include a nitrate, a sulfate, a carbonate, and a chloride. Ofthese, examples of the nitrate include lithium nitrate, sodium nitrate,potassium nitrate, cesium nitrate, and silver nitrate. Examples of thesulfate include lithium sulfate, sodium sulfate, potassium sulfate,cesium sulfate, and silver sulfate. Examples of the carbonate includelithium carbonate, sodium carbonate, and potassium carbonate. Examplesof the chloride include lithium chloride, sodium chloride, potassiumchloride, cesium chloride, and silver chloride. These molten salts maybe used alone or may be used in combination of plural kinds thereof.

Specifically, for example, in order to form the stress distributionpattern 1 on the glass surface side, it is preferred to use a KNO₃ saltas the molten salt, and in order to form the stress distribution pattern2 on the glass internal side, it is preferred to use a NaNO₃ salt as themolten salt. In addition, in the production of a conventional chemicallystrengthened glass, a mixed salt of NaNO₃ and KNO₃ was frequently used.However, for chemical strengthening of a glass containing Li, it is morepreferred to use them without being mixed.

The chemical strengthening treatment (ion exchange treatment) can be,for example, performed by immersing the glass in a molten salt heated to360 to 600° C. for 0.1 to 500 hours. The heating temperature of themolten salt is preferably 350 to 500° C., and the immersing time of theglass in the molten salt is preferably 0.3 to 200 hours.

The chemically strengthened glass of the present invention can beobtained by performing a two-stage chemical strengthening treatmentunder a different condition from each other. For example, a chemicalstrengthening treatment is performed under a condition under which CSbecomes relatively low as the first-stage chemical strengtheningtreatment, and then, a chemical strengthening treatment is performedunder a condition under which CS becomes relatively high as thesecond-stage chemical strengthening treatment. Thus, an integrated value(total amount of the compressive stress values) of compressive stressgenerated in the compressive stress layer can be suppressed low whileincreasing CS on the outermost surface of the chemically strengthenedglass, and as a result, an internal tensile stress (CT) can besuppressed low.

In order to realize a high surface compressive stress and a large depthof a compressive layer, specifically, for example, it is preferred thatafter the stress distribution pattern 2 on the glass internal side isformed by the first-stage chemical strengthening, the stressdistribution pattern 1 on the glass surface side is formed by thesecond-stage chemical strengthening treatment. In the case where thepattern 1 is formed by the first-stage chemical strengthening treatment,on the occasion of forming the pattern 2 by the second-stage chemicalstrengthening treatment, there is a case where the form of the pattern 1formed at the first stage is collapsed, whereby the depth of thecompressive stress layer of the pattern 1 becomes large.

Specifically, examples of the condition for obtaining the chemicallystrengthened glass of the present invention include the followingcondition of the two-stage chemical strengthening treatment.

First-stage chemical strengthening treatment: A glass preferably havinga base composition containing 50 to 80% of SiO₂, 4 to 30% of Al₂O₃, 0 to15% of B₂O₃, 0 to 15% of P₂O₅, 0 to 20% of MgO, 0 to 20% of CaO, 0 to10% of SrO, 0 to 10% of BaO, 0 to 10% of ZnO, 0 to 10% of TiO₂, 0 to 10%of ZrO₂, 3 to 20% of Li₂O, 0 to 20% of Na₂O, and 0 to 20% of K₂O ision-exchanged with a molten salt preferably containing sodium nitrate atpreferably 425 to 475° C. for preferably 2 to 5 hours.

Second-stage chemical strengthening treatment: The glass after thefirst-stage chemical strengthening treatment is ion-exchanged with amolten salt preferably containing potassium nitrate at preferably 375 to450° C. for preferably 0.5 to 2 hours.

In the present invention, the treatment condition of the chemicalstrengthening treatment is not particularly limited to theaforementioned condition, and the appropriate condition of time,temperature, and the like may be selected taking into consideration theproperties and composition of the glass, the kind of the molten salt,and the like.

The chemically strengthened glass of the present invention isparticularly useful as a cover glass to be used in mobile devices suchas a mobile phone, a smart phone, a personal digital assistant (PDA),and a tablet terminal. Furthermore, it is also useful, not for purposeof carrying, as a cover glass for display devices such as a television(TV), a personal computer (PC), and a touch panel; walls of elevator;walls (whole face display) of an architecture such as a house and abuilding; a building material such as an window pane; a table top; aninterior of an automobile, an aircraft, and the like; and a cover glassthereof. In addition, it is useful in uses as non-tabular housings orthe like having a curved shape by bending or molding.

EXAMPLES

The present invention is hereunder described by reference to Examples,but it should be construed that the present invention is not limited bythese Examples.

Glasses of Examples 1 to 24 shown in Table 1 were prepared and evaluatedin the following manners. Examples 1 to 12 and Examples 20 to 24 areconcerned with working examples, and Examples 13 to 19 are concernedwith comparative examples.

Examples 1 to 14 and 20 to 24 (Preparation of Chemically StrengthenedGlass)

Glass sheets of Glasses 1 to 5 each having a composition shown in Table1 in mole percentage on an oxide basis were prepared by platinumcrucible melting. Glass raw materials that are generally used, such asan oxide, a hydroxide, a carbonate, and a nitrate, were appropriatelyselected and weighed so as to be 1,000 g as the resulting glass.Subsequently, the mixed raw material was put in a platinum crucible,then put into an electric resistance furnace at 1,500 to 1,700° C., andmelted for about 3 hours to undergo defoamation and homogenization.

TABLE 1 Glass 1 Glass 2 Glass 3 Glass 4 Glass 5 SiO₂ 68.97 70.00 63.6666.92 66.12 Al₂O₃ 9.00 10.00 10.68 9.99 11.19 MgO 6.00 5.00 5.69 4.303.10 CaO 0.00 0.00 0.20 0.20 0.20 TiO₂ 0.04 0.00 0.11 0.11 0.11 ZrO₂1.00 1.00 1.00 1.00 1.30 Li₂O 9.50 10.00 10.68 10.09 10.39 Na₂O 4.503.00 5.59 5.49 5.59 K₂O 1.00 1.00 1.50 1.20 1.50 Y₂O₃ 0.00 0.00 0.900.70 0.50 Tg (° C.) 550 586 553 555 558

The resulting molten glass was cast into a mold, kept at a temperatureof [glass transition temperature+50° C.] for 1 hour, and then cooled toroom temperature at a rate of 0.5° C./min, to obtain a glass block. Theresulting glass block was cut and ground, and finally, the both surfacesthereof were mirror-finished to obtain a sheet-form glass having adesired shape. A thickness t [mm] of the glass is shown in Table 1. Theresulting glass was subjected to the two-stage chemical strengtheningtreatment shown in each of Tables 2 and 3, to obtain chemicallystrengthened glasses of Examples 1 to 14 and Examples 20 to 24. A stressprofile of the chemically strengthened glass of Example 5 is illustratedin FIG. 1. FIGS. 3 to 7 illustrate stress profiles of the chemicallystrengthened glasses of Examples 20 to 24, respectively.

(Stress Profile)

With respect to the chemically strengthened glass, a stress value CS_(x)[MPa] of a portion with depth of x [μm] from the glass surface wasmeasured or calculated. For the evaluation of CS_(x) until the depth of30 μM from the surface layer of the glass, a surface stress meterFSM-6000, manufactured by Orihara Industrial Co., Ltd. was used, and forthe evaluation of CS_(x) from the depth of 30 μm to the depth of 300 μm,SLP1000 which is a measuring device utilizing scattered-lightphotoelasticity, manufactured by Orihara Industrial Co., Ltd. was used.

Subsequently, values of A₁, B₁, A₂, B₂, C, A₁/B₁, and A₂/B₂ weredetermined by approximating the profile of CS_(x) by an errorleast-squares method in a range of 0<x<3t/8 using the following function(I). The results are shown in Tables 2 and 3.

A₁erfc(x/B₁)+A₂erfc(x/B₂)+C  (I)

In the function (I), erfc is a complementary error function, andrelations of A₁>A₂, and B₁<B₂ are satisfied.

Examples 15 to 19

Examples 15 to 19 are results described on the basis of PTL 1 (US2015/0259244 A). These are corresponding to Sample b, Sample f, Sampleg, Sample i, and Sample j, respectively in PTL 1. Values of A₁, B₁, A₂,B₂, C, A₁/B₁, and A₂/B₂ were determined by digitalizing the stress valueprofiles (FIGS. 5c, 9b, 10b, 11b, 12b, and 13b ) described in PTL 1 withan image analysis software and approximating the digitalized stressvalue profiles using the function (I). The results are shown in Table 3.In addition, the stress profile of the chemically strengthened glass ofExample 15 is illustrated in FIG. 1.

(CS and DOL)

CS and DOL of the chemically strengthened glasses of Examples 1 to 24are shown in Tables 2 to 3. CS and DOL were evaluated using a surfacestress meter FSM-6000, manufactured by Orihara Industrial Co., Ltd. andSLP1000 which is a measuring device utilizing scattered-lightphotoelasticity, manufactured by Orihara Industrial Co., Ltd. The depthat which the stress value CS_(x) is zero was defined as DOL, and CS_(x)on the outermost surface layer was defined as CS.

(Four-Point Bending Test)

With respect to the chemically strengthened glasses obtained in Example5, Example 9, Example 13, and Example 14, the bending strength (unit:MPa) was measured by performing the four-point bending test under acondition of a lower spun of 30 mm, an upper spun of 10 mm, and acrosshead speed of 0.5 mm/min. The results are shown in Table 2. InTable 2, a blank column means that the evaluation was not performed.

(Pendulum Impact Test)

With respect to the chemically strengthened glasses obtained in Example5, Example 9, Example 13, and Example 14, the fracture energy wasmeasured using a pendulum impact tester under an evaluation condition ofthe following condition. The results are shown in Table 2. A diamondindenter (indenter angle of the facing angle: 160°) was installed in atip of a pendulum, and a sum of weights of the pendulum and the diamondindenter was adjusted to 300 g. The pendulum and the diamond indenterwere lifted up to a certain fixed angle and then immobilized, followedby releasing, thereby controlling the tip of the diamond indenter so asto move with a certain fixed kinematic energy.

In the pendulum movement, by disposing a glass sample in the vicinitywhere the kinematic energy of the diamond indenter became maximum suchthat the diamond indenter collided perpendicular against the glasssample and repeatedly colliding the diamond indenter against the glasssample while making the kinematic energy as a variable, a fragmentationprobability of the glass sample was evaluated. The kinematic energy ofthe diamond indenter when the fragmentation probability of the glasssample reached 50% was evaluated as the fracture energy. The results areshown in Table 2. In Table 2, a blank column means that the evaluationwas not performed.

TABLE 2 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Example 7 Glass composition Glass 1 First-stage Molten salt NaNO₃ 100%salt strengthening Temperature (° C.) 450 condition Time (h) 3 3 3 3 3 33 Second-stage Molten salt KNO₃ 100% salt strengthening Temperature (°C.) 400 400 400 425 425 425 425 condition Time (h) 1 1.5 2 0.5 1 1.5 2Thickness t (μm) 800 800 800 800 800 800 800 A₁ (MPa) 901 876 850 906804 828 813 B₁ (μm) 1.5 1.7 2.1 1.5 2.2 2.8 3.2 A₂ (MPa) 348 336 378 349380 321 287 B₂ (μm) 177 185 229 185 208 202 207 C (MPa) −89 −90 −124 −93−114 −95 −87 A₁/B₁ 608 524 405 585 367 293 252 A₂/B₂ 2 1.8 1.7 1.9 1.81.6 1.4 B₂/t 0.221 0.232 0.286 0.231 0.26 0.252 0.259 CS (MPa) 1160 11221104 1162 1070 1054 1012 DOL (μm) 144 147 161 147 155 153 155 Stressprofile Solid line in FIG. 1 Four-point bending strength (MPa) 778Pendulum test results Fracture energy (mJ) 119 Example 8 Example 9Example 10 Example 11 Example 12 Example 13 Example 14 Glass compositionGlass 1 First-stage Molten salt NaNO₃ 100% salt KNO₃ 100% saltstrengthening Temperature (° C.) 450 450 condition Time (h) 3 3 3 2.32.3 6 6 Second-stage Molten salt KNO₃ 100% salt NaNO₃ 100% saltstrengthening Temperature (° C.) 450 450 450 450 450 425 450 conditionTime (h) 0.5 1 1.5 1 0.5 4 6 Thickness t (μm) 800 800 800 500 650 800800 A₁ (MPa) 783 741 735 685 789 446 446 B₁ (μm) 2.3 3.3 4.2 4.0 2.510.4 10.4 A₂ (MPa) 349 363 273 271 300 231 203 B₂ (μm) 196 246 226 111157 125 151 C (MPa) −99 −128 −91 −96 −116 −47 −50 A₁/B₁ 337 221 173 171309 43 43 A₂/B₂ 1.8 1.5 1.2 2.5 1.9 1.8 1.3 B₂/t 0.245 0.307 0.283 0.2210.242 0.156 0.189 CS (MPa) 1033 975 916 956 1089 629 610 DOL (μm) 151165 160 90 122 120 133 Stress profile Four-point bending strength (MPa)835 378 356 Pendulum test results Fracture energy (mJ) 128 64.0 64.0

TABLE 3 Example 15 Example 16 Example 17 Example 18 Example 19 Glasscomposition PTL 1 First-stage Molten salt Na52K48 salt Na45K55 saltNa37K63 salt Na38K62 salt Na37K63 salt strengthening Temperature (° C.)441 450 440 450 440 condition Time (h) 10 8.5 8.8 7.5 11 Second-stageMolten salt Na1K99 salt Na1K99 salt Na1K99 salt Na2K98 salt Na1K99 saltstrengthening Temperature (° C.) 390 390 390 390 390 condition Time (h)12 12 12 18 12 Thickness t (μm) 400 700 800 900 1000 A₁ (MPa) 563 599566 567 561 B₁ (μm) 6.5 6.6 6.6 6.8 6.9 A₂ (MPa) 432 359 438 451 447 B₂(μm) 72 80 76 89 89 C (MPa) −70.6 −66.4 −82.2 −92.9 −90.1 A₁/B₁ 87 90 8583 81 A₂/B₂ 6 4.5 5.7 5.1 5 B₂/t 0.181 0.115 0.096 0.099 0.089 CS (MPa)862 843 858 856 854 DOL (μm) 71 73 71 71 80 Stress profile Dotted linein FIG. 1 Example 20 Example 21 Example 22 Example 23 Example 24 Glasscomposition Glass 2 Glass 3 Glass 4 Glass 5 First-stage Molten saltNa100 Na10K90 Na100 Na100 Na10K90 strengthening Temperature (° C.) 450450 450 450 450 condition Time (h) 2 3 1 1 2 Second-stage Molten saltKNO₃ 100% salt strengthening Temperature (° C.) 425 380 415 400 380condition Time (h) 1 0.5 1 1 0.5 Thickness t (μm) 600 600 600 600 600 A₁(MPa) 778 945 992 1026 725 B₁ (μm) 3.0 3.5 1.3 1.7 3.8 A₂ (MPa) 385 344347 326 404 B₂ (μm) 134 125 126 134 117 C (MPa) −102 −87 −86 −87 −94A₁/B₁ 258 272 793 588 190 A₂/B₂ 2.9 2.8 2.8 2.4 3.5 B₂/t 0.224 0.2080.210 0.223 0.195 CS (MPa) DOL (μm) Stress profile FIG. 3 FIG. 4 FIG. 5FIG. 6 FIG. 7

As shown in Tables 2 and 3, it was noted that the chemicallystrengthened glasses of Examples 1 to 12 and 20 to 24 that are concernedwith working examples have both a high surface compressive stress and alarge depth of a compressive stress layer as compared with thechemically strengthened glasses of Examples 13 to 19 that are concernedwith comparative examples.

In addition, as illustrated in FIG. 1, it is noted that the chemicallystrengthened glass of Example 5 that is concerned with a working examplehas such a stress profile that the depth of a compressive stress layerof the stress distribution pattern 1 on the glass surface side is smalland that the compressive stress value of the stress distribution pattern2 on the glass internal side is small, and has both a high surfacecompressive stress and a large depth of a compressive stress layer ascompared with the chemically strengthened glass of Example 15 that isconcerned with a comparative example, while controlling the total amountof the compressive stress values to a certain value or less.

Furthermore, as shown in Table 1, it is noted that as compared withExample 13 and Example 14 each having a small surface compressive stressand small depth of a compressive stress layer, Example 5 and Example 9each having large surface compressive stress and large depth of acompressive stress layer are large with respect to the four-pointbending strength and the fracture energy in the pendulum impact test.

These results demonstrate that the chemically strengthened glass of thepresent invention has a high strength as a chemically strengthened glassas compared with that of the conventional art.

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof. The presentapplication is based on a Japanese patent application filed on Apr. 6,2017 (Japanese Patent Application No. 2017-076106) and a Japanese patentapplication filed on Feb. 5, 2018 (Japanese Patent Application No.2018-018508), the entireties of which are incorporated by reference. Inaddition, all the references cited herein are incorporated as a whole.

1. A chemically strengthened glass satisfying A₁ [MPa] of 600 or more,A₂ [MPa] of 50 or more, B₁ [μm] of 6 or less, B₂ [μM] of 10% or more oft [μm], C [MPa] of −30 or less, and A₁/B₁ [MPa/μm] of 100 or more whenthe chemically strengthened glass has a thickness t [μm] and a profileof a stress value [MPa] at a depth x [μm] from a glass surface isapproximated by an error least-squares method in a region of 0<x<3t/8using the following function (I) while defining a compressive stress aspositive and a tensile stress as negative:A₁erfc(x/B₁)+A₂erfc(x/B₂)+C wherein erfc is a complementary errorfunction, and relations of A₁>A₂ and B₁<B₂ are satisfied.
 2. Thechemically strengthened glass according to claim 1, wherein B₂ [μm] is20% or more of t [μm].
 3. The chemically strengthened glass according toclaim 1, wherein A₂ [MPa] is 150 or more, and A₂/B₂ [MPa/μm] is 4 orless.
 4. The chemically strengthened glass according to claim 1, whereinC [MPa] is −70 or less.
 5. The chemically strengthened glass accordingto claim 1, wherein the thickness t is 0.3 mm or more and 2 mm or less.6. The chemically strengthened glass according to claim 1, having a basecomposition comprising 50 to 80% of SiO₂, 4 to 30% of Al₂O₃, 0 to 15% ofB₂O₃, 0 to 15% of P₂O₅, 0 to 20% of MgO, 0 to 20% of CaO, 0 to 10% ofSrO, 0 to 10% of BaO, 0 to 10% of ZnO, 0 to 10% of TiO₂, 0 to 10% ofZrO₂, 3 to 20% of Li₂O, 0 to 20% of Na₂O, and 0 to 20% of K₂O in molepercentage on an oxide basis.
 7. The chemically strengthened glassaccording to claim 1, having a base composition comprising 50 to 80% ofSiO₂, 4 to 30% of Al₂O₃, 0 to 15% of B₂O₃, 0 to 15% of P₂O₅, 0 to 20% ofMgO, 0 to 20% of CaO, 0 to 10% of SrO, 0 to 10% of BaO, 0 to 10% of ZnO,0 to 10% of TiO₂, 0 to 10% of ZrO₂, 3 to 20% of Li₂O, 0 to 20% of Na₂O,0 to 20% of K₂O, and 0.1 to 5% of Y₂O₃ in mole percentage on an oxidebasis.
 8. The chemically strengthened glass according to claim 1, whichis a glass substrate for a cover glass.